The introduction of the catalytic cracking process in the oil industry at the end of the thirties was a determinant progress with respect to the prior techniques, by providing for highly improved yields of high-grade motor gasoline. The various processes operating in fixed bed (e.g. HOUDRY processes) have been rapidly supplanted by moving bed processes and, particularly since the middle of the forties, by those of the fluid bed type (fluid catalytic cracking, in short FCC). At the very beginning, the catalytic cracking processes were used nearly exclusively for treating vacuum distillates (VD) of low sulfur content that are relatively light (final boiling point lower than 540.degree.-560.degree. C.).
The cracking of these charges is generally conducted at about 500.degree. C. under a total pressure close to atmospheric pressure and in the absence of hydrogen. In these conditions the catalyst becomes quickly covered with coke and it is constantly necessary to regenerate it. In cracking processes of the fluid bed type (FCC) or of the moving bed type (such as TCC) the catalyst permanently circulates between the reaction zone where it resides about a few seconds to several tens of seconds, and the regenerator where it is freed of coke by combustion between about 600.degree. and 750.degree. C. in the presence of diluted oxygen.
The fluid bed (FCC) units are now used more extensively than those of the moving bed type. The catalyst circulates therethrough in a fluidized state as particles of average diameter ranging from 50 to 70 microns, the granulometry of said powder ranging approximately from 20 to 100 microns.
The catalysts used in the first FCC units were solids of high silica content obtained by acid treatment, either of natural clays or of synthetic silica-aluminas. The main progress in FCC achieved up to the end of the fifties was, in particular:
the use of the spray drying technology for preparing catalysts in the form of fine spherical particles more easily fluizidable and more resistant to attrition than the powder obtained by crushing, PA0 the synthesis of silica-aluminas, initially containing a high (about 85% by weight) silica and a low alumina (Lo-Al) proportion and then a high alumina (Hi-Al) content, with about 75% of SiO.sub.2, PA0 and various very substantial improvements concerning metallurgy and equipment design, particularly for regeneration. PA0 cracking in the riser (tube wherein the catalyst and the charge flow upwardly), PA0 decrease of the contact time, PA0 modification of the regeneration techniques. PA0 to obtain thermally and hydrothermally more stable catalysts, also more tolerant of metals, PA0 to reduce the amount of coke formation at equal conversion rate, PA0 to obtain a gasoline of higher octane number, PA0 to improve the selectivity to middle distillates. PA0 1. Cracking heavy molecules with a good selectivity to C.sub.3 hydrocarbons, particularly propylene, PA0 2. Sufficiently resisting to the severe conditions of steam partial pressure and of temperature prevailing in a regenerator of an industrial cracking unit. PA0 erionite of T erionite: these zeolites have a good selectivity for producing C.sub.3 hydrocarbons, but their hydrothermal stability is clearly not so good as that of offretite and, in addition, they suffer from the disadvantage of having too closed a pore structure, making them inefficient for the conversion of certain bulky hydrocarbon molecules, such for example as isoparaffins, naphthenes and alkylaromatics. PA0 ZSM5: this zeolite, hydrothermally very stable, is insufficiently selective for the production of C.sub.3 hydrocarbons, as shown in one of the examples given hereinafter. PA0 Ferrierite: this zeolite is hydrothermally very stable, but its pore structure is clogged at high temperature, in the presence of steam, by extensive extraction of aluminum atoms from the aluminosilicate structure. As a matter of fact, these extracts are housed in the micropores, thus blocking the access to the molecules, and they cannot be substantially removed by chelating or acid treatments. Moreover, when it still has a slight activity, ferrierite leads to a too large proportion of C.sub.1 or C.sub.2 hydrocarbons to be of interest. PA0 Mordenite: this zeolite is hydrothermally very stable and easy to stabilize and to dealuminate, but its selectivity for the production of C.sub.3 hydrocarbons is clearly insufficient. PA0 1--the hexagonal mesh of the offretite has a size along c axis which is one half of that of erionite (BENNET J. M. & GARD J. A., Nature 214, 1005, 1967), and also the odd lines 1 of erionite X-ray diffraction spectra are absent from the offretite spectra (GARD J. A. & TAIT J. M., Molecular Sieve Zeolites-1, Advan. Chem.,Ser. 101, 230, 1971); PA0 2--the piling sequences of the two zeolites are different (WHYTE T. E. Jr., WU E. L., KERR G. T. & VENUTO P. B., J. Catal. 20, 88, 1971). Thus the offretite has a much more open structure than erionite. Piling defects may occur in these structures, giving rise to the formation of T errionite, which is a zeolite of offretite structure with piling defects of the erionite type. PA0 (a) 20-95%, preferably 30-80% and advantageously 50-80% by weight of at least one matrix (constituent A), PA0 (b) 1-60%, preferably 4-50% and more advantageously 10-40% by weight of at least one zeolite of open structure other than offretite (constituent B); according to the present invention, the term zeolite of open structure designates a zeolite whose main dodecagonal channels have an opening of such a size that it is equivalent to a circular opening of at least 7 Angstrom diameter, PA0 (c) 0.5-60%, preferably 1-40% and more advantageously 2-30% of at least one offretite whose dodecagonal channels have an opening lower than 7 Angstroms (constituent C), having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio from about 15 to about 500, crystalline parameters of elementary mesh a and c such that a ranges from 1.285 to 1.315 nm and c from 0.748 to 0.757 nm and a potassium content lower than 1.5% by weight, the total alkali metal content being preferably lower than 1.5% by weight. PA0 light gases (hydrogen, C.sub.1 -C.sub.2 hydrocarbons), PA0 propylene, PA0 propane (C.sub.3), PA0 saturated C.sub.4 and iso-C.sub.4 hydrocarbons, PA0 C.sub.4 unsaturated hydrocarbons, PA0 gasolines, PA0 light cycle oil or light diluent (L.C.O.), PA0 a heavy cycle oil or heavy diluent (H.C.O.), PA0 a residue or slurry, generally freed of catalyst particles, for obtaining a clarified oil (C.O.) or a decanted oil (D.O.). PA0 SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio higher than about 15 and preferably higher than about 20 (particularly from 15 to 500, and more particularly from 20 to 300), PA0 crystalline parameter a ranging from about 1.285 to 1.315 nm, preferably from about 1.290 to about 1.310 nm, and crystalline parameter c from about 0.748 to about 0.757 nm, PA0 potassium content lower than 1.5% by weight and preferably lower than 0.5% by weight, and in addition: PA0 nitrogen adsorption capacity, at 77 K and at a P/P.sub.o ratio of 0.19, higher than 0.15 cc liquid per gram and preferably higher than 0.20 cc liquid per gram, PA0 cyclohexane adsorption capacity, at 25.degree. C. and at a P/P.sub.o ratio of 0.25, higher than 3 and preferably higher than 4% by weight, PA0 water adsorption capacity, at 25.degree. C. for a P/P.sub.o ratio of 0.1, lower than 15% and preferably lower than about 10% by weight. PA0 (a) 20-95%, preferably 30-80% and more advantageously 50-80% by weight of at least one matrix (constituent A), PA0 (b) 1-70%, preferably 4-60% and more advantageously 10-50% by weight of at least one zeolite of open structure other than a zeolite of the erionite family (constituent B). It is recalled that the term zeolite of open structure, as used in the present invention, means a zeolite whose main dodecagonal channels have an opening of such a size as to be equivalent to a circular opening of at least 7 Angstrom diameter, PA0 (c) 0.05-40% preferably 0.1-30% and more advantageously 0.5-10% by weight of at least one zeolite of the erionite family (offretite, ZSM-34, AG2, N-O, ZKU or T erionite for example) having a potassium content lower than 4% by weight, the total alkali metal content being preferably lower than 4% by weight (constituent C).
It is only at the beginning sixties that a major advance took place in the field of catalytic cracking, by the use of molecular sieves and more particularly of the zeolite of the faujasite structure, first in a moving bed process, then, a little later, in the FCC process. These zeolites, incorporated with a matrix mainly consisting of amorphous silica-alumina and optionally containing variable proportions of clay, are characterized by cracking activities for hydrocarbons about 1000 to 10000 times those of the first catalysts used. The availability on the market of these new zeolite catalysts has completely changed the cracking process by the very substantial gain of activity and of selectivity to gasoline and also by considerable modifications in the unit technology, particularly:
These three points will be further examined hereinafter.
The X zeolite (faujasite structure) characterized by a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio from 2 to 3 has been first used. Exchanged to a large extent with rare-earth ions, it is highly active and has a high thermal and hydrothermal resistance.
Towards the end of the sixties, this zeolite has been progressively replaced by Y zeolite which tends to produce slightly less coke and whose thermal and hydrothermal resistance was much improved. Presently, a major part of the proposed catalyst (probably more than 90%) contain an Y zeolite exchanged with rare-earth ions and/or ammonium ions.
From the beginning of the sixties the oil industry began to suffer from a shortage of available crude oil, whereas the demand for gasoline of high octane number was continuously increasing. Moreover, the available supply was progressively oriented towards heavier and heavier crude oils. The treatment of the latter raised difficult problems for the refiner in view of their high content of catalyst poisons, particularly nitrogenous compounds and metal compounds (mainly nickel and vanadium), their exceptional Conradson carbon and overall asphaltene compound values.
The necessity to treat heavier charges and other more recent problems such as: the progressive but general elimination of lead-containing additives, the slow but substantial evolution in various countries of the demand for middle distillates (kerosene and gas-oil), have induced the refiners to make searches for finding improved catalysts whereby the following objects can be met:
It is mostly desirable to reduce the production of light gases comprising compounds of 1-4 carbon atoms and accordingly the catalysts are so designed as to limit the production of such light gases. But in certain countries, particularly in certain developing countries, the demand for these products or for some of them, particularly propylene, may be high. The catalytic cracking process may meet to a certain extent such a demand, provided that the catalyst be particularly adapted to said production.
An efficient way for adapting the catalyst consists of adding to the conventional catalyst masses an active agent having the two following properties of:
As a matter of fact, in view of the tendency of the present charges to produce more and more coke which deposits onto the catalysts, and of the higher sensitivity to coke of zeolite performance, the present research has as an object not only finding catalysts less selective to coke, but also to further catalyst regeneration in order to reduce to a minimum the coke amount at the end of the combustion. In many processes, this is achieved by increasing the regenerator temperature.
Consequently, the regenerator is subjected to high steam partial pressures, ranging from 0.2 to 1 bar (1 bar=0.1 MPa), and to local temperatures at the catalyst level of 750.degree.-850.degree. C. or even 900.degree. C., for a few tens of seconds to a few minutes. In these conditions the zeolite, which is the main active agent of the catalyst, may rapidly lose a large part of its activity due to the irreversible degradation of its structure. In spite of various artful techniques developed during the last years for limiting the regenerator temperature (addition of coils to remove heat by producing steam or intermediary cooling of the catalyst) or for limiting the steam content at high temperature (a technique using two regenerators as that used in the R2R process of TOTAL-IFP), the zeolite present in the cracking catalyst must necessarily have an excellent thermal and hydrothermal stability.